Twelve volt electrical systems for automobiles have become an industry standard. As the quest for improved vehicle performance and occupant safety has intensified, the importance of electrical systems in automobiles has become pivotal. Electronic ignitions and diagnostic engine monitoring computers have increased engine performance and efficiency. Anti-lock brakes, traction control, and air-bags have increased occupant safety in automobiles.
Although the importance and complexity of electrical systems and their components has increased, the basic electrical generation and storage mechanisms have remained relatively unchanged. Typically, a twelve volt automobile system is comprised of an alternator, a 12 volt, six cell, lead acid storage battery, a starter motor, an ignition system, and a variety of other electrical loads.
The alternator generates the power required by the electrical system during operation of the vehicle. The alternator takes the mechanical energy of the engine and produces current to charge a 12 volt lead acid battery and to supply the remaining electrical loads. The battery serves as a power source to start the engine and to power the electrical system when the engine is not running. While the engine is running, the alternator re-charges the battery and provides the primary source of current to the electrical system.
The various loads on the electrical system include fans, heaters, lights, pumps, and motors. Usually, the component with the largest current demand is the starter motor. The starter motor provides torque to the crankshaft to start the engine. Accordingly, the battery must be properly sized to meet the current requirements of the starter motor. Some weather conditions, e.g. during extremely cold weather, require higher starting torque. Higher torque may also be required due to certain engine modifications; for example, racing engine modifications.
In order to achieve better engine performance, racing automobile engines are commonly modified to increase the valve compression ratio by reducing the volume of the combustion chamber by special machining. An increase in the compression ratio results in higher torque being required to turn the crank shaft when the motor is started. Higher torque is particularly beneficial when restarting a hot high-compression racing engine. In a Direct Current (DC) starter motor, the current drawn varies in proportion to the mechanical torque applied to the motor output shaft.
The increased torque required to start a highly tuned racing engine results in a proportional increase in the DC current required to operate the starter motor effectively. This increased current, flowing in the starter circuit of the vehicle, causes an increase in the voltage drop due to resistive dissipation in the cables, starter motor armature windings, and in the internal resistance of the vehicle battery. To offset the increased voltage drop, some racing automobiles are fitted with specially designed 16 volt batteries having eight cells.
In addition to the requirement for increased starting torque, it may be desirable to increase the operating speed and performance of other electro-mechanical components such as fluid pumps, solenoids, relays, and other electrical components. Improved performance of the engine in horsepower and torque may result from a hotter spark in an ignition using 16 volts.
There is, however, a disadvantage to having a battery with a single higher output voltage. Voltage charging levels as high as 20 volts are required to recharge a 16 volt battery. Many electrical components, particularly electrolytic capacitors, integrated circuits, and instrumentation, are sensitive to voltage increases and could be damaged by the higher voltage. To avoid these undesired effects, a three terminal battery has been developed which has a common negative terminal and both 12 and 16 volt positive terminals. Therefore, electrical components of the automobile can be powered selectively from either voltage as required for optimum performance.
Lead acid storage batteries are the most commonly used batteries in automobiles. These batteries must be recharged in order to replace the energy taken from the battery and maintain the specific gravity of the liquid electrolyte. Conditions of under-charging or over-charging a lead acid battery will reduce the effectiveness and the expected life of the battery. Lead acid batteries are typically recharged at a voltage of between 2.3 volts and 2.5 volts per cell, therefore the charge voltage required by the battery depends upon the number of cells in the battery. The standard charging system installed in most automobiles is designed to charge 12 volt (6 cell) batteries, and cannot easily be modified to charge 16 volt (8 cell) batteries.
Conventional battery charging schemes for recharging dual voltage batteries, or batteries with increased voltage levels, required the use of special external AC powered 16 volt battery chargers or 16 volt voltage alternators. These charging schemes utilize a single higher voltage output across the ground and 16 volt terminals of the battery. Since discharging of the lower six (6) cells may occur at a different rate than the higher two (2) cells, a cell imbalance will occur. Over a number of charge and discharge cycles, the cell imbalance will result in the lower six (6) cells becoming completely discharged and the upper two (2) cells being continually overcharged in an attempt to charge the whole battery.
There is an example of a multiple voltage battery charging scheme that does not use high voltage alternators or external AC chargers disclosed in U.S. Pat. No. 4,723,079 (Norton). In Norton, a scheme is utilized in which a generator supplies a plurality of voltage levels through a series of voltage regulators. Such charging systems are relatively costly and/or complex.